The present invention is a new method for preparing pavement-marking sheet material.
Pavement-marking sheet material which is economical and performs well in both daylight and night conditions is a continuing goal of the pavement-marking industry. Night performance is primarily provided by retroreflection, which may be defined as a phenomenon in which a large portion of luminous radiation is returned in the direction from which it originates. Spherical retroreflectors, typically tiny glass beads or microspheres, are well known in the industry.
Flat single layer, polymeric sheet material, as well as flat laminated materials, are known to hold the beads in position. The sheet material is applied to a highway surface and serves to both cushion and hold the retroreflective beads. The efficiency of flat pavement-marking materials, however, is limited for two reasons. First, the exposed surfaces of the beads are directed upward, whereas the optimal orientation is toward vehicle headlights which typically illuminate the retroreflective beads from angles slightly above the road surface. Second, in an upwardly directed fashion, the exposed surfaces of the beads are exposed to maximum abrasive wear by vehicle tires, thus allowing rapid destructive abrasion of the exposed surface.
An alternative approach is to provide a raised pattern on the pavement marking strip. A raised pattern includes three advantages. First, a raised pattern encourages runoff of rain water. Second, a raised pattern provides nonhorizontal surfaces to support retroreflective beads. The nonhorizontal surface isolates the optical surfaces of the beads from abrasive wear by traffic and provides a more effective orientation of the retroreflective beads. Third, a raised pattern allows the use of highly efficient specular retroreflecting beads.
U.S. Pat. No. 4,069,281 discloses a prefabricated roadway marking strip material including a base layer and a traffic-regulating, sign-forming and traffic wear-resisting upper layer having a generally smooth surface. Spaced protuberances bulge from the surface and retroreflective elements are concentrated on the top portion of the protuberances. The protuberances are thick portions of the generally smooth upper layer.
Another road marking material is described in U.S. Pat. No. 3,935,365. The material is formed by embossing spaced transverse protrusions on extruded stock. Beads are placed on each transverse protrusion to provide reflectorized flanks.
An alternative product is disclosed in U.S. Pat. No. 4,681,401 (Wyckoff). The product includes flattened, somewhat saw-toothed shaped wedges embodying retroreflective material and of preferably substantially trapezoidal shape.
Another method of forming a raised pattern on a pavement-marking sheet material is disclosed in U.S. Pat. No. 4,388,359. The preparation of the material includes depositing a monolayer of microspheres on a base sheet and embossing the microsphere-covered base sheet so as to deform the base sheet and form protuberant areas separated by depressed areas. The embossing step partially embeds the glass microspheres into the base sheet in the protuberant areas and can fully embed the microspheres into the base sheet into the depressed areas.